Only read first 512 bytes

[kevinsawicki]

Since only the first 512 bytes are considered, only read the first 512 bytes instead of the entire file.

This improves performance noticeably for files in the hundreds of megabytes:

> ls -al /tmp/huge.zip
-rw-r--r--  1 kevin  wheel  237545472 May 24 12:24 /tmp/huge.zip

> git co master 
> time bin/isbinaryfile /tmp/huge.zip
real    0m0.250s
user    0m0.067s
sys     0m0.183s

> git co read-first-512-only
> time bin/isbinaryfile /tmp/huge.zip
real    0m0.101s
user    0m0.065s
sys     0m0.035s

[gjtorikian]

ah, yeah, I've only ever used this in the context of the file already being read. 😺

mergin'.